Terminology

Spectroscopy is the detailed study of the light from an object.
Spectrometers are instruments which spread light out into its
wavelengths creating a spectrum. Within this spectrum, astronomers can
study emission and absorption lines which are the fingerprints of
atoms and molecules. An emission line occurs when an electron drops
down to a lower orbit around the nucleus of an atom and looses
energy. An absorption line occurs when electrons move to a higher
orbit by absorbing energy. Each atom has a unique spacing of orbits
and can emit or absorb only certain energies or wavelengths. This is
why the location and spacing of spectral lines is unique for each atom.
Astronomers can learn a great deal about an object in space by
studying its spectrum, such as it's composition (what its made of),
temperature, density, and it's motion (both it's rotation as well as
how fast it is moving towards or away from us).

What is the reddening law?

A "reddening law" is really just a description of how interstellar dust
absorbs light that passes through it. When starlight passes through a dust
cloud, not only does it get dimmer (because some of the light is being
absorbed), it also gets "redder" because the shorter wavelength light is
affected more than the longer wavelength light. Interstellar clouds are
more efficient at scattering and absorbing blue light than red light, so
much less blue light gets through them. That makes stars appear redder
when seen through a dust cloud. The "reddening law" allows astronomers to
correct this effect, and infer what the star would look like without the
dust getting in the way. Different astronomers use different reddening
laws, depending on who they think has the better model of exactly how much
light is absorbed at different wavelengths. Sometimes this creates
arguments and controversy, as no one knows what the right law really
is. And getting the right version of the law is important, as it allows
astronomers to estimate the actual distances and temperatures of the stars
in question.

What is absolute zero?

At a temperature of Absolute Zero there is no motion and no heat.
Absolute zero is where all atomic and molecular motion stops and is the
lowest temperature possible.
Absolute Zero occurs at 0 degrees Kelvin or -273.15 degrees Celsius or
at -460 degrees Fahrenheit.
All objects emit thermal energy or heat unless they have a temperature of absolute zero.

What are degrees Kelvin and Celsius and how do they relate to degrees Fahrenheit?

In the early years of the eighteenth century, Gabriel Fahrenheit
(1686-1736) created the Fahrenheit scale.
He set the freezing point of water at 32 degrees and the boiling point at
212 degrees. These two points formed the anchors for his scale.

Later in that century, around 1743, Anders Celsius (1701-1744)
invented the Celsius scale. Using the same anchor points, he determined
the freezing temperature for water to be 0 degree and the boiling
temperature 100 degrees. The Celsius scale is known as a Universal
System Unit. It is used throughout science and in most countries.

There is a limit to how cold something can be. The Kelvin scale is
designed to go to zero at this minimum temperature. The relationships
between the different temperature scales are:

K = 273.15 + °C
°C = (5/9)*(°F-32)
°F = (9/5)*°C+32

What is the difference between luminosity, absolute
magnitude and apparent magnitude?

The luminosity of an object in space is the amount of energy that it radiates
each second in all directions.
Luminosity is also referred to as the absolute
magnitude or absolute brightness of an object.
It is the real brightness of a celestial object.
The apparent magnitude or apparent brightness of an object is a measure of how
bright an object appears to be to an observer. It is the amount of energy
from an object in space which reaches a square centimeter of a detector each
second. Apparent magnitude is also referred to as flux.
It is a measure of how bright a celestial object appears to us. The apparent
magnitude of an object depends upon its real brightness and on its distance
from us.

What is an Angstrom?

An angstrom is a unit of distance which is commonly used to measure the
wavelength of light. 1 Angstrom = 10-10 meters = 0.000,000,000,1
meters. For example, visible light ranges from about 4000 Angstroms (blue) to
about 7000 Angstroms (red).

What is an Astronomical Unit?

An astronomical unit (A.U.) is the average distance between the Earth and the Sun,
which is about 93 million miles or 150 million kilometers. Astronomical units
are usually used to measure distances within our solar system. For example,
the planet Mercury is about 1/3 of an A.U. from the Sun, while the farthest planet,
Pluto, is about 40 A.U. from the Sun (that's 40 times as far away from the Sun as the
Earth is).

What is a light-year?

Most objects in space are so far away, that using a relatively small unit of
distance, such as an astronomical unit, is not practical. Instead, astronomers
measure distances to objects which are outside our solar system in light-years.
A light-year (ly) is the distance that light can travel in one year in a vacuum
(empty space). The speed of light is about 186,000 miles or 300,000 kilometers
per second. So, in one year light travels a distance of about
5,880,000,000,000 miles or 9,460,000,000,000 kilometers or 63,240 A.U.. This
distance is 1 light-year. For example, the nearest star to us is about 4.3
light-years away.
Our galaxy, the Milky Way, is about 150,000 light-years across, and the nearest
large galaxy, Andromeda, is 2.3 million light-years away.

What are arcminutes and arcseconds?

In astronomy, we often use angular measurements to describe the apparent size of an
object in space and the apparent distances between objects. Often these angles
are very small.
Angles are also used to describe an object's location in space.
The angular measure of an object is usually expressed in degrees, arcminutes or
arcseconds. Just as an hour is divided into 60 minutes and a minute into
60 seconds, a degree is divided into 60 arcminutes and an arcminute is divided
into 60 arcseconds. To give you an idea of how small an arcsecond is, imagine
the width of a dime as seen from 2 kilometers or 1 1/4 miles away.

To get a rough estimate of the angular size of objects in space, you can go
out on clear night when the moon is up. Extend your arm towards the sky.
Your fist, at arms length, covers about 10 degrees of the sky, your thumb covers
about 2 degrees, and your little finger covers about 1 degree.
If you look at the Moon, it should take up about 1/2 a degree in the sky.
The Big Dipper should be about 20 degrees (two fists at arms length) from
one end to the other.

What is spatial resolution?

The spatial resolution of a telescope depends on the size of its lenses or mirrors
and the size of the pixels in its detectors. The resolution is also limited by air
turbulence (for ground based observatories) and by the smoothness of a telescope's
mirrors or lenses. The spatial resolution of a telescope
is proportional to the
wavelength of light being detected divided by the diameter
of the telescope. Larger telescopes have better spatial resolution. However,
it is the size of the telescope relative to the wavelength that really counts.
The longer the wavelength, the larger the telescope needs to be to get good
resolution.

What is the sensitivity of a telescope?

The sensitivity of a telescope is the smallest signal that it can
clearly measure from a source in space.
It is the minimum brightness that a telescope can detect.
A telescope with high sensitivity can detect very dim objects, whereas a
low sensitivity telescope can only detect the brighter objects in space.
Many objects in space are very dim as seen from the Earth. Some are naturally
dim objects because they do not emit much light. Others only appear to be dim
because they are a great distance from us. It is important for a telescope to
have the greatest sensitivity possible, so that it can observe the
many different types of objects in space.